1. Field of the Invention
The present invention relates generally to techniques for filtering signals, and more particularly, to techniques to eliminate discontinuities in adaptively filtered signals.
2. Related Art
In digital speech communication involving encoding and decoding operations, it is known that a properly designed adaptive filter applied at the output of the speech decoder is capable of reducing the perceived coding noise, thus improving the quality of the decoded speech. Such an adaptive filter is often called an adaptive postfilter, and the adaptive postfilter is said to perform adaptive postfiltering.
Adaptive postfiltering can be performed using frequency-domain approaches, that is, using a frequency-domain postfilter. Conventional frequency-domain approaches disadvantageously require relatively high computational complexity, and introduce undesirable buffering delay for overlap-add operations used to avoid waveform discontinuities at block boundaries. Therefore, there is a need for an adaptive postfilter that can improve the quality of decoded speech, while reducing computational complexity and buffering delay relative to conventional frequency-domain postfilters.
Adaptive postfiltering can also be performed using time-domain approaches, that is, using a time-domain adaptive postfilter. A known time-domain adaptive postfilter includes a long-term postfilter and a short-term postfilter. The long-term postfilter is used when the speech spectrum has a harmonic structure, for example, during voiced speech when the speech waveform is almost periodic. The long-term postfilter is typically used to perform long-term filtering to attenuate spectral valleys between harmonics in the speech spectrum. The short-term postfilter performs short-term filtering to attenuate the valleys in the spectral envelope, i.e., the valleys between formant peaks. A disadvantage of some of the older time-domain adaptive postfilters is that they tend to make the postfiltered speech sound muffled, because they tend to have a lowpass spectral tilt during voiced speech. More recently proposed conventional time-domain postfilters greatly reduce such spectral tilt, but at the expense of using much more complicated filter structures to achieve this goal. Therefore, there is a need for an adaptive postfilter that reduces such spectral tilt with a simple filter structure.
It is desirable to scale a gain of an adaptive postfilter so that the postfiltered speech has roughly the same magnitude as the unfiltered speech. In other words, it is desirable that an adaptive postfilter include adaptive gain control (AGC). However, AGC can disadvantageously increase the computational complexity of the adaptive postfilter. Therefore, there is a need for an adaptive postfilter including AGC, where the computational complexity associated with the AGC is minimized.